Per [1] and the warnings our CI is emitting GitHub is phasing in
"macos-12" as their "macos-latest".
As with [2], let's pin our image to a specific version so that we're
not having it swept from under us, and our upgrade cycle can be more
predictable than whenever GitHub changes their images.
1. https://github.com/actions/runner-images/issues/6384
2. 0178420b9c (github-actions: run gcc-8 on ubuntu-20.04 image,
2022-11-25)
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
d11192255d